NO,Sonic should be the fastest character in Brawl. That's his trademark,you can't that away.
Anyway,he should be unlockable in the last event match where you play as Mario and fight him on Final Destination. I soon as I got Sonic I wouldn't even play as him.
This is what I would do(in this order) if the blue blur was in the game: Have an earth-shattering nerdgasm over the fact that Sonic is in Brawl, set Mario and Sonic on computer mode,both in level 9(or whatever the CPU level will be) plop them in Final Destination,sit back,relax,get ready for the ultimate battle,press start,and enjoy the show.